With the multi-detector, the D16 diffractometer can be used for a wide range of measurements, including small-angle scattering, reflectometry, and long-wavelength diffractometry. Conventionally each multi-detector exposure is stored in a separate file, and identified with a run number. Treatment of data requires the possible merging of a number of data files, for each sample, and also background measurements. The out-of plane information is less well defined because the beam is usually vertically focussed to lead neutrons from the whole height of the guide-tube onto the sample. In the horizontal plane the monochromator mosaicity and slits before the sample restrict the overall incident beam divergence to about 15 minutes; if required this can be further reduced using soller collimators.
The detector efficiency is usually measured using a uniform scatterer. Typically a 1mm water sample is used with the detector at a few degrees offset from the direct beam. By dividing the measured spectrum by the water spectrum the relative cell by cell efficiency and the solid angle subtended by each cell are taken into account. If no normalising run is give then the efficiency is taken as unity everywhere.
The program red16 can treat one or a sequence of frames (which may have consecutive or uniformly spaced run numbers).
The data in each frame are added into a resultant spectrum, taking account of any changes in the detector angle. The multi-detector data can be added using an equatorial projection (which simply corresponds to adding the data in columns of y), or azimuthally, which takes into account the scattering on cones centred at the sample. Individual cell contents are allocated to adjacent uniform mesh values using fractional linear interpolation from the cell position.
Regrouping is made in equi-angle steps; by default the x-pixel width is converted to the angular step width.
Parameters and efficiency data which have been changed are written to disk and are re-used the next time the program is run.
The output listing includes the statistical error in the intensity per cell, and also the weighted deviation for the contributing cells. For a uniform distribution these are similar; large differences indicate sample ordering giving rise to anisotropy.
The program uses single letter keywords to designate operations. Optional parameters are shown in square brackets.
R run1 [run2] [da] [dr] Regroup data
Regroups data for runs run1 to run2, with a step of dr to allow for repetitive non-contiguous sequences. The angular step, da degrees, overwrites the default angular step derived from the x-pixel width.
The present version of the program uses the monitor of the first run of a sequence to re-ratio subsequent scan components. Thus if measurements are made using constant time intervals a small correction is made. A table of count rates and the ratios of components flux to that of the first scan is shown. If these show strong deviations from constancy then this should be further investigated. Normally the reactor flux is constant to better than 1%.
Including sets of runs
R [I] [T] run1 [run2] [da] [dr]
Successive use of RI commands add the runs or sequences to a list for treatment together. If the letter T is included then this terminates the list and treatment commences.
If, after starting a list an error is found, then the command R alone will reset the number of runs to zero and no treatment will be performed.
L run1 run2 lset [da] >
This will reduce data for linked sequences of runs length lset starting at run1 and saving the reduced data as SANS data files. The next sequence is a set of the same length starting at run1+lset-1, etc., if this is less than or equal to run2.
N run ext low high Normalisation data
Takes the data from run/ext and assumes this is a flat spectrum to produce a file of efficiency dividing factors with an average value of unity. If the limit levels are non-zero, the normalised factors searched, only including pixels within values, and the normalisation corrections are recalculated for these remaining data. These dividers are stored. If a zero run number is given then unit efficiency is assigned to all cells.
Using other programs from the SANS suite, e.g. anpoly, effdef and rmask it is possible to perform more complex masking than the simple window included in the program.
D r1 r2 r3 r4 .. Raw runs from a nominally isotropic sample
The raw data are summed and a normalisation factors for the detector efficiencies are calculated and stored for subsequent use. Up to ten runs may be included.
B x0 y0 g0 sd wav ovr Beam centre, s-d distance, wavelength
The incident beam characteristics are set with this command, namely the beam centre, x0 and y0 (in cells) when the detector angle is at angle g0, the sample-detector distance (in metres), and the wavelength in Angstroms. The beam centre is used as an offset subtracted from the value found in the data. If the wavelength or the sample-detector distance in the data are zero then the values here are used. If ovr is set to 1 then the wavelength and s-d values here are used. (Note that ovr is not stored and the next program run will default to the standard where the data override the values input manually here).
M maskfile Reads in a maskfile
A maskfile constructed with standard SANS programs rmask, grund etc can be read in and used to designate cells to be eliminated from regrouping calculations. A blank filename eliminates the mask option. Note: the name and extension must be given; both are case-sensitive.
A [A] or [E] Averaging mode for regrouping - azimuthal averaging
or equatorial projection
This command sets the type of regrouping procedure to be used.
G [H] [V] or [S] or [M} Detector Geometry (1K) Horizontal or Vertical
or Square for the 16K detector or M for the MILAND 320x320 detector
This command sets the appropriate cell dimensions and orientation.
F filename Ascii file of Angle/Counts/Error/Pixels
The regrouped data from the R command are written into a file "filename". Three header information lines are followed by the number of points and then four columns of angle, counts, the standard error in the counts, and the number of contributing pixels.
S save data in ILL-SANS format Q/Counts/Error
The regrouped data are stored in standard SANS format and may be treated with programs from this suite.
P [H] Plot data as Angle/Counts/Error
The regrouped data are plotted within the limits set by the X and Y commands. If the letter H is included then the data are written out for printing (typically as a PostScript file, though this depends on the setting of the environment variable PGPLOT_ILL_DEV_2)
Q [H] Plot data as Q/Counts/Error
The regrouped data are plotted within the limits set by the X and Y commands. If the letter H is included then the data are written out for printing (typically as a PostScript file).
X [min] [max] shows or sets the X scale limits for plots
Y [min] [max] shows or sets the Y scale limits for plots
C toggles cursor off or on in the next plot
Clicking in the graphics window, or pointing and typing spaces marks the coordinates of the cursor on the graph. These markers are reproduced on hardcopy if the next plotting command includes an H. The end of cursor input is signalled by typing RETURN while the pointer is in the graphics window.
W [R] [x1 x2 y1 y2] show or [Re]sets data window to be used
The rectangle defined by the coordinates x1,x2,y1,y2 only is used for regrouping. This is a simple way of eliminating the outside regions, if these are considered to contain unreliable data. The first and last rows and columns are always excluded from all regrouping.
I r1 [low high] [H] [R] displays detector image
This is shown within the low and high limits if set. The run number is incremented, so a second I command would show the next run. Normally the efficiency corrected and masked data are shown. If R is also given in the command then the raw data are shown. Adding H to the command produces a hardcopy file. Giving low a negative value resets automatic scaling within the data limits.
V shows parameters set for current treatment
Z resets all options to default values
Option R,L,N,D,B,P,M,A,G,Q,S,F,X,Y,W,I,V,Z HELP or EXIT :
red16>p h
Option R,L,N,D,B,P,M,A,G,Q,S,F,X,Y,W,I,V,Z HELP or EXIT :
red16>e
Closing listing file d16r637.lis
%PGPLOT, Closing file d16r638.ps
is1 14% plo d16r638.ps
request id is hp_cs-1191 (1 file(s))
